Materials comprising cholesteric liquid crystals, also referred to as “chiral nematic” liquid crystals, are capable of maintaining a plurality of different optical states in the absence of an electrical field. Additionally, the optical state of the cholesteric liquid-crystal material can be changed from one state to another in response to applied electrical and/or thermal fields. These properties make these materials useful in the development of field-stable, rewritable displays.
In particular, cholesteric liquid-crystal materials are capable of being electrically driven, at ambient temperatures, between a reflective planar state (reflecting a specific visible wavelength of light) and a light-scattering focal-conic state. Cholesteric liquid-crystal materials have the capacity of maintaining these two optical states, planar or focal-conic, in the absence of an electric field. For example, U.S. Pat. No. 5,437,811 issued Aug. 1, 1995 to Doane et al. discloses a light-modulating cell having a polymer-stabilized chiral-nematic liquid-crystal material that is capable of switching between a planar state, reflecting a specific visible wavelength of light, and a weakly light-scattering focal-conic state.
U.S. Pat. No. 5,636,044 discloses a bistable cholesteric display. Two patterned substrates, made of glass or plastic, face each other. Cholesteric material is disposed between the two substrates or plates. The cholesteric material can contain a polymer gel or dye. Electrodes are exposed by offsetting the substrates to expose connection areas on the substrates. The display is built by bonding the two substrates together and then filling the cell with liquid-crystal material, after which radiation is applied to create polymer threads in the display that stabilize the cholesteric material. Cholesteric material processed in such a manner is known as a polymer stabilized cholesteric (PSC). Such displays require two substrates.
U.S. Pat. No. 4,140,016 discloses a plurality of selectively deposited cholesteric materials disposed on a substrate to create a temperature sensing paddle. The cholesteric materials are encapsulated using closed-core microencapsulation. The materials can be deposited by a variety of processes such as gravure printing, silk screen printing, and the like. There are no electrodes in the structure that permit an electric field to be applied across the cholesteric material. Such materials change state only in the presence of a specific temperature, and cease to maintain the second state in the absence of an specific temperature.
Fabrication of flexible, electronically written display sheets is disclosed in U.S. Pat. No. 4,435,047 issued Mar. 6, 1984 to Fergason. An emulsion of nematic liquid crystal in water is coated over a plastic sheet having a low-resistance ITO coating. A doctor blade is used to cast the emulsion over the sheet at a specific thickness. The liquid crystal material is a nematic liquid crystal with a dye that can be electrically switched between a transparent and light-blocking state. The display ceases to present an image when de-energized. The coated electrode is unpatterned, and contacted by a single electrical lead. No mention is made as to how the first electrode is kept free of coated materials that are coated over the first conductor.
U.S. Pat. No. 5,289,300 discloses a liquid-crystal material formed over a semiconductor array. The material is a UV-cured polymer-dispersed cholesteric liquid-crystal material. Coating methods disclosed include solvent coating of the polymer, including water and hydrocarbon solvents, using methods including doctor blades or roll coating. No methods are disclosed that describe how the inner electrodes are clear of the polymer-dispersed overcoat.
U.S. Pat. No. 6,262,697 discloses a coated polymer-dispersed liquid-crystal layer. An inner electrode is buried under the polymer-dispersed material. The author discloses the use of a piercing pin to form connection to the inner electrodes. U.S. Pat. No. 6,236,442 discloses another means for connecting to an inner conductor coated with polymer-dispersed liquid-crystal material. Overcoated layers are removed to expose a power area that permits connection to an inner transparent, electrically conductive layer.
It would be useful to have a process and structure to improve the manufacture of a display in which a polymer-dispersed cholesteric material is built-up on a substrate. It would be advantageous for the process not to require the removal of previously coated layers.